More than 20 organizations have online calculators that estimate your personal share of the heat-trapping gases tied to global warming. Once they give you a ballpark figure, the organizations tell you how much you'd need to donate to ecofriendly projects to prevent or stop companies from emitting a comparable amount.

How harmful is air travel?

No one knows for sure. Aviation may account for 2 percent of greenhouse-gas emissions. But most climate scientists believe that air travel has added warming effects because of jet contrails and nitrogen oxide emissions. And when considered by energy use per mile covered, planes eat up far more energy than buses and trains.

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Are offsets affordable?

For the typical domestic trip, an offset costs less than $20 per person. For example, to offset a ½-ton of carbon dioxide, which is approximately what would be produced by a round-trip flight between Chicago and Miami, you could buy an offset for $6 from TerraPass.